The smell of a mouthwash that repeatedly precedes the experience of a painful dental procedure first begins to trigger a conditioned anxiety response during the process of

The smell of a mouthwash during the process of ACQUISITION.
In psychology, acquisition refers to the first stage of learning when a response is established, that is, it refers to the period when a particular stimulus consistently evoke a particular conditioned response.
In the question given above, the smell of a mouthwash is the stimulus which brings about the experience of a painful dental procedure.